Harnessing discontinuous phase transitions in polyelectrolyte hydrogels for mesoscale filament assembly

Materials composed of complex linked topologies, including knits and weaves, exhibit properties unattainable through other structural forms. However, developing materials with these topologies at the mesoscale, or smaller, remains challenging due to limited manipulation strategies. Here, we describe the fabrication of poly(acrylic acid) copolymer polyelectrolyte hydrogel mesoscale filaments and the characterization of their complex, reversible, spatiotemporal actuation, which exploits discontinuous swelling/deswelling phase transitions in electric fields. When both ends are fixed to a substrate, the filaments exhibit a winding actuation due to electroosmotic flow induced by an applied electric field. Field polarity governs the flow direction, providing temporal control over the actuation through polarity switching, and spatial patterning of the electric field determines actuation locations. We characterize the kinetics, equilibrium, and reversibility of filament actuation behaviors as functions of copolymer composition and field strength. The actuation speed and extent of deformation are found to be highly controllable since field strength determines the drift velocity of free protons and thus the electroosmotic flow rate. This also governs the configuration of charged polymer chains, hence influencing the mechanical properties of the hydrogels across compositions. This work establishes a platform for dynamic filament actuation and mesoscale hierarchical assemblies.